I don't see developer convenience and user experience as mutually exclusive. If an application is simpler and more consistent, that can save time for both users

I don't see developer convenience and user experience as mutually exclusive. If an application is simpler and more consistent, that can save time for both users and developers. (I also don't see how a one-process-per-DOM model prevents any other forms of sandboxing inside the browser. The OS doesn't understand webpages and cannot isolate them from each other - the browser does - so sandboxing pages is the browser's responsibility...)